The present invention relates generally to improved feedthrough terminal pin sub-assemblies and related methods of construction, particularly of the type used in active implantable medical devices, to decouple and shield undesirable electromagnetic interference (EMI) signals from the device. More particularly, the present invention relates to a reduced cost and reduced mechanical stress hermetic feedthrough terminal pin and ceramic feedthrough capacitor assembly which does not require an outer electrical connection between the one or more capacitors and a ground plane which is typically found in the form of a ferrule or device housing ground. It is adapted particularly for use in connecting one or more lead wires or conductive terminal pins through a hermetically sealed housing to internal electronic components of the medical device while decoupling EMI against entry into the sealed housing. The present invention is specifically designed for use in active implantable medical devices, such as cardiac pacemakers, implantable defribrillators, hearing implants, neuro-stimulators, drug pumps, bone growth stimulators, and the like.
Feedthrough terminal pin assemblies are generally well known in the art for connecting electrical signals through the housing or case of an electronic instrument. For example, in implantable medical devices such as cardiac pacemakers, defibrillators or the like, the terminal pin assembly comprises one or more conductive terminal pins supported by an insulator structure for feedthrough passage from the exterior to the interior of the medical device. Many different insulator structures and related mounting methods are known in the art for use in medical devices wherein the insulator structure also provides a hermetic seal to prevent entry of body fluids into the housing of the medical device. Said hermetic insulators for medical implant applications are typically constructed of alumina ceramic or glass wherein the terminal pins and ferrule are of suitable biocompatible material such as platinum, platinum-iridium, niobium, tantalum or titanium, respectively. However, the feedthrough terminal pins are typically connected to one or more exterior lead wires, for example, the leads which connect a cardiac pacemaker to the ventricle chamber of the heart, can also effectively act as an antenna and thus tend to collect stray EMI signals for transmission into the interior of the medical device. In many prior art devices, the hermetic terminal pin assembly has been combined directly with a ceramic feedthrough filter capacitor to decouple interference signals to the housing of the medical device.
In a typical unipolar construction, as described in U.S. Pat. No. 5,333,095 (the contents of which are incorporated herein), a coaxial ceramic feedthrough filter capacitor used in a feedthrough assembly to suppress and decouple undesired interference or noise transmission along a terminal pin comprises a so-called discoidal capacitor having two sets of electrode plates embedded in spaced relation within an insulative dielectric substrate or base, formed typically as a ceramic monolithic structure. One set of the electrode plates is electrically connected at an inner diameter cylindrical surface of the discoidal capacitor structure to the conductive terminal pin utilized to pass the desired electrical signal or signals. The other or second set of electrode plates is coupled at an outer diameter surface of the discoidal capacitor to a cylindrical ferrule of conductive material, wherein the ferrule is electrically connected in turn to the conductive housing of the electronic device. The number and dielectric thickness spacing of the electrode plate sets varies in accordance with the capacitance value and the voltage rating of the discoidal capacitor. In operation, the discoidal capacitor permits passage of relatively low frequency biological electrical signals along the terminal pin, while shielding and decoupling/attenuating undesired interference signals of typically high frequency to the conductive housing.
Feedthrough capacitors of this general type are available in unipolar (one), bipolar (two), tripolar (three), quadpolar (four), pentapolar (five), hexpolar (six) and additional lead configurations. The feedthrough capacitors of this general type are commonly employed in implantable cardiac pacemakers, defibrillators, and the like, wherein the pacemaker housing is constructed from a biocompatible metal, such as titanium alloy, which is electrically coupled to the second electrode plate set of the feedthrough filter capacitor. As a result, the filter capacitor and terminal pin assembly prevents entrance of interference signals to the interior of the pacemaker housing, wherein such interference signals could otherwise adversely affect the desired cardiac pacing or defibrillation function.
In the past, feedthrough filter capacitors for cardiac pacemakers and the like have typically been constructed by preassembly of the discoidal capacitor within a cylindrical or rectangular terminal pin subassembly which includes the conductive pin and ferrule. More specifically, the terminal pin subassembly is prefabricated to include one or more conductive terminal pins supported within the conductive ferrule by means of a hermetically sealed insulator ring or bead. See, for example, the subassemblies disclosed in U.S. Pat. Nos. 3,920,888; 4,152,540; 4,421,947; and 4,424,551. An improved design which has substantially improved the volumetric efficiency is based upon surface mounting of a ceramic feedthrough capacitor planar array structure to one outer surface of a hermetic terminal with similar connection to the conductive pins (see the subassemblies disclosed in U.S. Pat. No. 5,333,095). In all of the prior art described above, the outer feedthrough capacitor electrode plate sets are coupled in parallel together by a metallized layer which is either fired, sputtered or plated onto the ceramic capacitor. This metallized band, in turn, is coupled to the ferrule by conductive adhesive, soldering, brazing, or the like.
Although feedthrough filter capacitor assemblies of the type described above have performed in a generally satisfactory manner, the manufacture and installation of such filter capacitor assemblies has been relatively time consuming and therefore costly. For example, installation of the discoidal capacitor into the small annular space described by U.S. Pat. No. 4,424,551 between the terminal pin and ferrule can be a difficult and complex multi-step procedure to ensure formation of reliable, high quality electrical connections. The method taught by U.S. Pat. No. 4,424,551 (the contents of which are incorporated herein), teaches the injection of fluidic thermosetting conductive particles into first and second annular cavities (usually by centrifuge operations). As a consequence, this method also requires insulation of the interface between the capacitor structure and insulator, curing of the various thermosetting materials, and subsequent cleaning operations to remove excess conductive material. While the method taught by U.S. Pat. No. 5,333,095 is far simpler, a connection from the capacitor outside diameter and the conductive ferrule is still required.
A significant advance in the state of the art is described by internally grounded feedthrough capacitors as described in U.S. Pat. Nos. 5,905,627 and 6,529,103. These patents describe a methodology wherein it is not necessary to form a direct electrical connection between the outside diameter metallization of the feedthrough capacitor and the ferrule of the hermetic terminal of the implantable medical device. The internal ground technology teaches grounding of the second set of electrode pins through one or more grounded terminal pins. This has a significant advantage in that the elimination of the electrical and mechanical connection from the capacitor outside diameter or perimeter acts to significantly reduce the thermal and mechanical stress that is transmitted to the rather fragile feedthrough capacitor during the installation of the hermetic terminal pin assembly into the housing of the active implantable medical device by laser welding or the like. A significant number of EMI filtered hermetic terminals used in cardiac pacemakers and implantable defibrillators are manufactured using an internally grounded feedthrough capacitor. This has become a very popular and cost effective way of manufacturing such terminals. However, a significant cost driver is the need to provide one or more grounded pins for coupling to the second set of capacitor electrode plates. The present invention describes a novel method of providing internally grounded electrode plates within the hermetic terminal insulator itself. As will be further described herein, such internal ground plates within the insulator can be used to conveniently ground one or more terminal pins to couple to the second electrode plate set of the internally grounded capacitor.
A high integrity hermetic seal for medical implant applications is very critical to prevent the ingress of body fluids into the implanted device (e.g. pacemaker). Even a small leak rate of such body fluid penetration can, over a period of many years, cause moisture to build up and damage sensitive internal electronic components. This can cause catastrophic failure of the implanted device. The hermetic seal for medical implant applications is typically constructed of highly stable alumina ceramic or glass materials with very low bulk permeability.
Withstanding the high temperature and thermal stresses associated with the welding of a hermetically sealed terminal with a premounted ceramic feedthrough capacitor is very difficult to achieve with the prior art designs. The electrical/mechanical connection to the outside perimeter or outside diameter of the feedthrough capacitor has a very high thermal conductivity as compared to air. The hermetic bonding operation typically employed in the medical implant industry to install the filtered hermetic terminal into the implantable device housing generally involves a laser welding operation in very close proximity to this electrical/mechanical connection area. Accordingly, in the prior art, the ceramic feedthrough capacitor is subjected to a dramatic temperature rise. This temperature rise produces mechanical stress in the capacitor due to the mismatch in thermal coefficients of expansion of the surrounding materials. Many of these prior art devices employ a soldered connection to the outside perimeter or outside diameter of the feedthrough capacitor. Excessive installation soldering heat has been known to damage such devices.
The novel internally grounded feedthrough capacitors that are described by U.S. Pat. Nos. 5,905,627 and 6,529,103 solve these issues. By elimination of the outside diameter or outside perimeter electrical/mechanical connection area, the thermal and mechanical stresses are greatly reduced. That is, during laser welding of the titanium flange into the housing of the implantable medical device, said titanium flange will tend to expand and contract greatly. This transmits stresses to the rather sensitive monolithic ceramic feedthrough capacitor. Accordingly, elimination of said mechanical and electrical connection is a highly desirable feature. The present invention describes a very novel and convenient way of making connection to the second set of electrode plates without creating additional mechanical stresses.
A major market force within the medical implantable device industry has been to reduce the cost of the implanted device (e.g. pacemaker or implantable cardioverter defibrillator). Medical insurance carriers, government healthcare programs (e.g. Medicare) and health maintenance organizations (HMOs) are placing additional competitive pressures on the manufacturers of such devices.
Accordingly, there is a need for a novel feedthrough filter capacitor assembly that addresses the drawbacks noted above in connection with the prior art. In particular, a novel capacitor assembly is needed that is subjected to far less temperature rise during the manufacture thereof by eliminating an outside perimeter or outside diameter electrical/mechanical connection. Such a design would allow the use of much lower temperature materials (such as standard solder) to achieve the capacitor inside diameter lead connections. Moreover, such an improvement would make the assembly relatively immune to the aforementioned stressful installation techniques. A novel filter capacitor design is needed which is of simplified construction, utilizing a straightforward and uncomplicated feedthrough terminal pin subassembly that can result in manufacturing cost reductions. Of course the new design must be capable of effectively filtering out undesirable electromagnetic interference (EMI) signals from the target device. The present invention fulfills these needs and provides other related advantages.